1. Field of the Invention
This invention relates to a finder's optical system in a single lens reflex camera having a pentaroof prism for indicating information as to the proper exposure represented by the F-number, shutter speed, exposure meter needle, light-emitting diode, or the like, within the upper portion of the finder's view field.
2. Description of the Prior Art
Various types of optical systems for indicating information within the finder of a single lens reflex camera have been proposed and have their own merits and demerits.
The most typical method of presenting such indication is that which comprises disposing an indicator member on the focal plane of a focusing plate or between the focusing plate and the bottom surface of a pentaroof-prism, and illuminating the indicator member with the light from a picture-taking lens or any other extraneous light directed thereto in any manner, thereby indicating the information along the periphery of the finder's view field.
Apart from cases in which information is indicated within the finder's view field in superposed relationship with such view field, using the above-described method to indicate the information outwardly of the finder's view field entails a corresponding enlargement of the finder's view field, which could necessarily lead to a corresponding enlargement of the finder system itself and accordingly a relatively large pentaroof-prism.
Further, illuminating the indicator member with the light from the picture-taking lens would involve a serious problem in that the quantity of the effective illuminating light is reduced to provide a lower visibility of the indication itself as would be experienced when the lens is stopped down or when use is made of a lens such as a telephoto lens whose exit pupil is distant from the focal plane. Using any other extraneous light than the light from the picturetaking lens to illuminate the indicator member would also involve a design for preventing entry of the extraneous light into the finder itself, thus complicating the illumination.
In an effort to overcome these disadvantages, there is a method which comprises disposing an indicator member forwardly of the pentaroof-prism and indicating the information within the finder's view field through the pentaroof-prism and eyepiece. In this case, it is most natural and desirable that the indicator member be located at a position conjugate with the focal plane of the finder's view field so as to be viewed with a magnification and visibility equal to those of the view field. For this reason, considerable difficulties are often encountered in adjusting the length of the optical path from the eyepiece to the indicator member as by providing an optical path correcting lens or prism in the optical path.